Erase verify for non-volatile memory

ABSTRACT

A memory device verify system determines a state of memory cells in a memory. The memory includes a memory array having a plurality of memory cells coupled to bit lines. A verify circuit is coupled to the bit lines to determine if memory cells have a erase level that is within predetermined upper and lower limits. The verify circuit can include first and second comparators. In one embodiment, the first comparator is used to compare a bit line current with an upper first reference current. The second comparator is used to compare a bit line current with a lower second reference current. The comparator circuit is not limited to reference currents, but can use reference voltages to compare to a bit line voltage. The verify circuit, therefore, eliminates the need for separate bit line leakage testing to identify over-erased memory cells. Memory verify methods using two reference signal are also provided.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to memory devices and inparticular the present invention relates to determining erase levels ofmemory cells in non-volatile memory devices.

BACKGROUND OF THE INVENTION

The use of non-volatile memory systems that maintain data integrity whena power supply is removed are expanding rapidly in integrated circuittechnology. A class of non-volatile memory systems having memory cellswhich has a source, a drain, a channel, a floating gate over the channeland a control gate are widely used. Two popular types of non-volatilememory designs in this class is the electronically erasable andprogrammable read only memories (EEPROM) and the FLASHerasable-programmable read only memory (EPROM). The FLASH EPROM or flashmemory system allows the simultaneous erasure of multiple memory cells.

The floating gate of the memory cell stores data and the control gate ofthe memory cell controls the floating gate. The floating gates aregenerally formed from polysilicon members completely surrounded by aninsulator. A memory cell is programmed when a charge is stored on thefloating gate. Moreover, a memory cell is unprogrammed, or erased, whenthe charge is removed from the floating gate.

One method of programming a memory cell is accomplished by applying apotential (e.g., 4–7 V) to its drain and a potential (e.g., 10–15 V) toits control gate programs. This causes electrons to be transferred fromthe source to the floating gate of the memory cell. One method oferasing a memory cell is accomplished by applying a positive potential(e.g., 10–15 V) to its source while grounding the control gate andletting the drain float. This action removes electrons from the floatinggate.

A problem that may be encountered in erasing a memory cell isover-erasure. This occurs when too many electrons are removed from thefloating gate during an erase operation. A memory cell whose floatinggate has too many electrons removed is called an over-erased cell. Anover-erased cell has a slight positive charge that biases the memorycell thereby causing a small current leak. This current leak can cause afalse reading. Moreover, during the read mode, an over-erased memorycell may disable a whole column of memory cells in a memory array.Therefore, it is important to locate over-erased cells and correct them.One method of correcting an over-erased cell is accomplished by applyinga soft program that applies a predetermined voltage pulse to the controlgate of the cell while the bit line is biased. This action eliminatesthe slight positive charge on the floating gate.

Another problem that may be encountered is under-erased memory cells.Under-erased memory cells occur when not enough electrons are removedfrom the floating gate during an erase procedure. An under-erased memorycell is corrected by performing another erase procedure.

Currently two separate steps are taken to determine if a memory cell isover-erased or under-erased. First the memory cells are individuallychecked to determine if they are all erased. Once that step iscompleted, the memory cells are then checked to see if any cells havebeen over-erased by checking bit line leakage current. The completion ofboth steps takes a significant amount of time.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art fora system to determine over-erased and under erased cells using lessprocessing time.

SUMMARY OF THE INVENTION

The above-mentioned problems with memories and other problems areaddressed by the present invention and will be understood by reading andstudying the following specification.

In one embodiment, a non-volatile memory comprises an array ofnon-volatile memory cells arranged in columns using bit lines, and averify circuit selectively coupled to the bit lines to determine if thememory cells have a program level that is within a program level windowdefined by first and second reference signals.

In another embodiment, a bit line verify system comprises a firstcomparator to compare a bit line current with a first reference currentand produce a first output signal, and a second comparator to comparethe bit line current with a second reference current and produce asecond output signal.

A memory device with an erase verify system is provided in yet anotherembodiment. The memory device comprises a memory array having aplurality of memory cells coupled to a bit line, a first comparator tocompare a bit line voltage with a first reference voltage and produce afirst output signal, and a second comparator to compare the bit linevoltage with a second reference voltage and produce a second outputsignal.

A non-volatile memory comprises an array of non-volatile memory cellsarranged in columns using bit lines, and a verify circuit selectivelycoupled to the bit lines to determine if the memory cells have a programlevel that is within a program level window defined by first upper andsecond lower reference signals. Control circuitry is coupled to theverify circuit to determine that the memory cell is erased if the memorycell has a program level within the window. The control circuitrydetermines that the memory cell is over-erased if the memory cell has aprogram level greater than the first upper reference signal. The controlcircuitry determines that the memory cell is under-erased if the memorycell has a program level lower than the second lower reference signal.

A method of erase verifying a non-volatile memory cell is provided inone embodiment, the method comprises generating a first referencecurrent, generating a second reference current, and comparing a bit linecurrent from a column coupled to the non-volatile memory cell with thefirst reference current and the second reference current.

A method of erase verifying a non-volatile memory cell is provided inanother embodiment, the method comprises generating a first referencevoltage, generating a second reference voltage, and comparing a bit linevoltage from a column coupled to the non-volatile memory cell with thefirst reference voltage and the second reference voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a flash memory device of oneembodiment of the present invention that is coupled to an externalprocessor.

FIG. 2 is a schematic diagram of a memory array coupled to a verifycircuit of one embodiment of the present invention.

FIG. 3 is a block diagram of a verify circuit of one embodiment of thepresent invention.

FIG. 4 is a block diagram of a verify circuit of one embodiment of thepresent invention.

FIG. 5 is a block diagram of another embodiment of a verify circuit ofthe present invention.

FIG. 6 is a block diagram of another embodiment of a verify circuit ofthe present invention.

FIG. 7 is a schematic diagram of a bit line current-to-voltage converterof one embodiment of the present invention.

FIG. 8 is a schematic diagram of a reference current-to-voltageconverter of one embodiment of the present invention.

FIG. 9 is a schematic diagram of another embodiment of a referencecurrent-to-voltage converter of the present invention.

FIG. 10 is a schematic diagram of a reference current-to-voltageconverter of one embodiment of the present invention to provide multiplereference voltages.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings that form a part hereof,and in which is shown by way of illustration specific preferredembodiments in which the inventions may be practiced. These embodimentsare described in sufficient detail to enable those skilled in the art topractice the invention, and it is to be understood that otherembodiments may be utilized and that logical, mechanical and electricalchanges may be made without departing from the spirit and scope of thepresent invention. The following detailed description is, therefore, notto be taken in a limiting sense, and the scope of the present inventionis defined only by the claims.

FIG. 1 illustrates a block diagram of a flash memory device 100 that iscoupled to an external processor 102. The memory device 100 has beensimplified to

focus on features of the memory that are helpful in understanding thepresent invention. The memory device 100 includes an array 116 of memorycells. The memory cells are preferably floating gate memory cells, andthe array 116 is arranged blocks of rows and columns. The blocks allowthe memory cells to be erased in large groups. Data, however, is storedin the memory array 116 in small data groups (byte or group of bytes)and separate from the block structure. Erase operations are usuallyperformed on a large number of cells in parallel.

Address decode circuitry 112 is provided to decode address signalsprovided on address lines A0–Ax 114. Address signals are received anddecoded to access the memory array 116. Data input and output buffercircuits 122 are included for bi-directional data communication over aplurality of data (DQ) lines 124 with the external processor 102.Control circuit 130 decodes signals provided on control lines 126 fromthe external processor 102. These signals are used to control theoperations of the memory, including data read, date write, and eraseoperations, as known to those skilled in the art. Verify circuits 128are included for verifying the state of a memory cell, as described indetail below.

In addition, state machine(s) can be provided as part of the controlcircuitry to perform read, write and erase operations. The flash memorymay also include a charge pump (not shown) that generates an elevatedvoltage, Vpp, used during programming of the memory cells and otherinternal operations. During write operations, Vpp is coupled to thememory cells for providing appropriate write operation programmingpower. Charge pump designs are known to those skilled in the art, andprovide power which is dependent upon an externally provided supply ofvoltage Vcc.

As stated above, the flash memory of FIG. 1 has been simplified tofacilitate a basic understanding of the features of the memory. Further,it will be appreciated that more than one flash memory can be includedin various package configurations. For example, flash memory cards canbe manufactured in varying densities using flash memories.

A more detailed illustration of a flash memory array 130 is provided inFIG. 2. As FIG. 2 illustrates, the memory cells 110 are made up offloating gate transistors 132 that are arranged in a plurality of rowsand columns (only one column is illustrated in FIG. 2). In the memoryarray, the source regions 134 of each memory cell in a row are connectedto a common source line 136. The drain regions 138 of each memory cellin a column are connected to a common bit line 140. In addition, controlgates 142 of each memory cell 110 in a row are connected to a word line144. The array of FIG. 2 has been simplified to illustrate the basicarrangement of memory cells and bit lines. Those skilled in the art willappreciate that the schematic diagram has been simplified to focus onthe present invention and that additional rows and columns would beimplemented to create a complete memory device.

During an erase verify operation, a voltage is applied to word line 144of a memory cell 110. In response to the word line voltage, the memorycell conducts a current through bit line 140. That is, the memory cellresponds to the word line voltage based on a charge of floating gate146. The level of current in the bit line indicates a state of thememory cell. That is, the memory cell may have an erase state that iseither erased, over-erased or under-erased. An erase verify circuit 128,of one embodiment of the present invention, uses the bit line current todetermine if memory cells are erased, over-erase or under-erased in asingle step. As explained above, prior methods required a first eraseverify operation to determine if memory cells are erased. A secondoperation is then performed to determine if memory cells wereover-erased.

FIG. 3 illustrates a block diagram of the verify circuit 128 with a bitline input 140, two references current inputs and an output(s). Theverify circuit can be selectively coupled to a bit line 140 and firstand second reference currents, Ir1 and Ir2. The verify circuit comparesa bit line current to the two reference currents and provides an outputsignal that indicated if the bit line current is within a current windowdefined by two reference currents. In one embodiment, the verify circuitprovides multiple output signals.

The verify circuit 128 can includes a comparator circuit 150, asillustrated in FIG. 4. The comparator circuit 150 is coupled to the bitline 140 and indicates if the bit line current (Ib1) is within a currentwindow defined by the two reference currents Ir1 and Ir2. The comparatorcircuit 150 includes first and second comparators 152, 154. The firstcomparator 152 compares the bit line current (Ib1) with the firstreference current (Ir1) and produces a first output signal (Os1). Thesecond comparator circuit 154 compares the bit line current Ib1 with asecond reference current (Ir2) and produce a second output signal (Os2).The two output signals can be output from verify circuit 128 or theverify circuit can use the two output signals to determine a state ofthe bit line current. Sample outputs of the two comparators areillustrated in Table 1.

TABLE 1 Memory Operation Os1 Os2 Memory Cell State Ib1 < Ir1 0 0 NeedFurther Erase Ir1 < Ib1 < Ir2 1 0 Pass Erase Verify Ib1 > Ir2 1 1Over-Erase

For example, the first reference current (Ir1) may be set at 50 μA andthe second reference current (Ir2) may be set at 90 μA. A 40 μA window,therefore, is defined by these references. It should be noted that thesecurrent levels are only used as an example. The reference current levelsmay vary depending on defined specifications of the memory device beingused. According to this example, any current over 90 μA indicates thatthe bit line 140 is coupled to an over-erased cell and any current under50 μA indicates a current that would be found in a bit line 140 that wascoupled to a memory cell 110 that was under-erased. Referring to Table2, three possible bit line current (Ib1) levels and the two outputsignals are illustrated when the reference currents are set at 90 μA and50 μA.

TABLE 2 Bit line Current, Ib1 Os1 Os2 Memory Cell State  40 uA 0 0 NeedFurther Erase  70 uA 1 0 Pass Erase Verify 100 uA 1 1 Over-Erase

Current comparators and current references of FIG. 4 can be designed asshown in FIG. 5, with current to voltage converters and voltagecomparators. Referring to FIG. 5, a comparator circuit 150 compares abit line voltage 140 with reference voltages, Vr1 and Vr2. In thisembodiment, a current-to-voltage converter 160 is used to convert thebit line current to a bit line voltage (Vb1). The current-to-voltageconverter 160 is coupled between bit line 140 and first and secondvoltage comparators 155, 157. A current-to-voltage converter 200 iscoupled between a reference current input (Iref) and first and secondcomparators 155, 157. Thus, the current-to-voltage converter 200 providea first reference voltage (Vr1) and a second reference voltage (Vr2).

The first comparator 155 of the comparator circuit 150 compares the bitline voltage Vb1 with the first reference voltage Vr1 and produces afirst output signal (Os1). The second comparator 157 of the comparatorcircuit 150 compares the bit line voltage Vb1 with the second referencevoltage Vr2 and produces a second output signal (Os2). An optional logiccircuit 151 can be provided to process the output signals, Os1 and Os2,and provide a single output to indicate if the bit line has a voltagelevel within a window defined by Vr1 and Vr1. The logic circuit 151 canbe included with verify circuit 128. That is, the embodiments of FIGS.3, 4, 5 and 6 can each comprise logic circuit 151.

FIG. 6 illustrates an alternate embodiment having first and secondcurrent to voltage converters 162 and 164. The first and secondcurrent-to-voltage converters provide reference voltages, Vr1 and Vr2,in response to reference currents, Ir1and Ir2, respectively. Referringto Table 3, three possible bit line voltage (Vb1) levels and the twooutput signals are illustrated.

TABLE 3 Bit Line voltage Vb1 Os1 Os2 Memory Cell State Vb1 > Vr2 0 0Need Further Erase Vr1 < Vb1 < Vr2 1 0 Pass Erase Verify Vb1 < Vr1 1 1Over-Erase

One embodiment of bit line current-to-voltage converter 160 isillustrated in FIG. 7. The bit line current-to-voltage converter 160includes a resistor 170 and an activation circuit 172. The activationcircuit 172 is used to provide a current path through the resistor. Theactivation circuit can include an activation transistor 174 and aninverter 182. Resistor 170 is coupled to the drain 176 of the activationtransistor 174. Inverter 182 is coupled between gate 178 of theactivation transistor 174 and source 180 of the activation transistor174. In addition, source 180 of the activation transistor 174 is furthercoupled to bit line 140. During operation, the bit line current Ib1pulls the input of inverter 182 low. The inverter then activatestransistor 174 to provide a current path through resistor 170. A voltagedrop across the resistor establishes the bit line voltage, Vb1. Thevoltage output, Vb1, of the bit line current-to-voltage converter 160can be determined by the following equation: Vb1=Vcc−R(Ib1). While thiscurrent-to-voltage converter uses the bit line current to establish theoutput voltage, similar converters can be used to provide referencevoltages.

Referring to FIGS. 8, 9 and 10, three embodiments of current-to-voltageconverter circuits are described that can be used to provide referencevoltages. One embodiment of a reference current-to-voltage converter162, 164 is illustrated in FIG. 8. The converter includes a resistor171, an activation circuit 173 and a reference current source 190. Thecircuit operates in a manner similar to converter 160, but usesreference current source 190 to establish the voltage drop acrossresistor 171 to provide Vr1 or Vr2. In another embodiment illustrated inFIG. 9, control current source 190 comprises a floating gate transistor194 that has been programmed to conduct a specific current in responseto a control voltage. Thus, when the control voltage is coupled to thecontrol gate of transistor 194, a reference current flows throughactivation circuit 173.

Referring to FIG. 10, one embodiment of a dual reference voltageconverter circuit 200 is described. Converter 200 provides two referencevoltage outputs Vr1 and Vr2 from a single reference current Iref. Theconverter includes a first resistor (R1) 202, a second resistor (R2)204, an activation circuit 172 and a reference current circuit 190. Asexplained above, control current source 190 can comprise a non-volatilememory cell in one embodiment. The first resistor R1 and the secondresistor R2 are coupled in series with the activation circuit 173 andthe reference current circuit 190. When a current is conducted throughthe resistors, the first reference Voltage Vr1 and the second referencevoltage Vr2 are determined by the following equations: Vr1=Vcc−(R1)(I),and Vr2=Vcc−(R1+R2)(I).

As explained above, the memory includes control circuitry 130 to performread, program and erase operations on the memory array. The controlcircuit uses the output(s) of the verify circuit to determine a state ofmemory cells being erased in one operation step. Thus, if an over-erasedcell is detected the control circuitry performs a soft program, or healoperation, to correct over-erased cells. Moreover, if an under-erasedcell is detected the control circuitry performs an additional eraseprocedure.

A typical erase algorithm for a standard stacked one transistor flashcell includes three main phases: 1) pre-program to program all cells; 2)erase to apply erase pulses to the cells and verify until cells areerased; and 3) heal to detect cell leakage and apply a program scheme toover-erased cells. The present invention, the leakage detection step ismerged with the verify portion of phase 2. As erase verification isperformed, the system determines if the cell is over-erased. The celladdress and status information can be latched for use in phase 3, or canbe used immediately by applying the heal programming scheme to thatcell, column or array.

CONCLUSION

An erase verify system has been described that determines a state ofmemory cells in a non-volatile memory. The memory includes anon-volatile memory array having a plurality of memory cells coupled tobit lines. A verify circuit is coupled to the bit lines to determine ifmemory cells have a erase level that is within predetermined upper andlower limits. The verify circuit can include first and secondcomparators. The first comparator is used to compare a bit line currentwith an upper first reference current. The second comparator is used tocompare a bit line current with a lower second reference current. Thecomparator circuit is not limited to reference current, but can usereference voltages and a bit line voltage. The verify circuit,therefore, eliminates the need for separate bit line leakage testing toidentify over-erased memory cells. Methods of detecting a bit linecurrent have also been described.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement, which is calculated to achieve the same purpose,may be substituted for the specific embodiment shown. This applicationis intended to cover any adaptations or variations of the presentinvention. Therefore, it is manifestly intended that this invention belimited only by the claims and the equivalents thereof.

1. A non-volatile memory comprising: an array of non-volatile memorycells arranged in columns using bit lines; and a verify circuitselectively coupled to the bit lines to determine if the memory cellshave an erase level that is within an erase level window defined byfirst and second reference signals, the circuit generating an indicationof one of under-erasure, erasure, or over-erasure in response to aposition of the erase level with the erase level window.
 2. Thenon-volatile memory of claim 1 wherein the verify circuit comprises: afirst comparator with a reference input coupled to receive the firstreference signal, and a second input coupled to the selected one of thebit lines; and a second comparator with a reference input coupled toreceive the second reference signal and a second input coupled to theselected one of the bit lines.
 3. The non-volatile memory of claim 1wherein the memory cells comprise a plurality of floating gate memorycells.
 4. The non-volatile memory of claim 1 wherein the indication is a“00” for under-erasure, a “10” for erasure, and a “11” for over-erasure.5. The non-volatile memory of claim 1 wherein the indication is alogical indication.
 6. The non-volatile memory of claim 1 wherein theerase level window indicates a current level.
 7. The non-volatile memoryof claim 6 wherein the first and second reference signals are currentsignals.
 8. A non-volatile memory comprising: an array of non-volatilememory cells arranged in a row and column format such that the columnscomprise bit lines including a selected bit line having a bit linecurrent; and a plurality of comparators for generating indicationsignals in response to a comparison of the bit line current with aplurality of reference currents each coupled to a comparator, eachreference current indicating a different limit of an erase level window,each comparator generating an indication of one of under-erasure,erasure, or over-erasure in response to the comparison.
 9. Thenon-volatile memory of claim 8 wherein the plurality of comparatorscomprise a first comparator that compares the bit line current to alower limit reference current and a second comparator that compares thebit line current to an upper limit reference current.
 10. Thenon-volatile memory of claim 9 wherein a bit line current that is lessthan the upper limit reference current and greater than the lower limitreference current is a current indicating erasure.